Conventionally known is the so-called tandem-type color laser printer having photosensitive drums forming electrostatic latent images, developing rollers developing the electrostatic latent images and toner boxes accommodating toners fed to the developing rollers, arranged correspondingly to four colors, i.e. yellow, magenta, cyan and black, respectively.
The tandem-type color laser printer forms generally simultaneously toner images of the respective colors on the respective photosensitive drums and sequentially transfers the toner images of the respective colors from the photosensitive drums to a sheet sequentially passing through the photosensitive drums, and therefore can form color images at a speed generally identical to that of a monochromatic laser printer.
For example, there has been proposed a tandem-type color laser printer having image forming stations, each including a photosensitive member, a developing unit having a developing roller and storing a developing agent and a transfer unit, provided correspondingly to respective colors, and forming color images by passing a transfer medium through the image forming stations of the respective colors.
In the tandem-type color laser printer, a set of four photosensitive members as well as a coroner charger and a cleaner arranged around the photosensitive member can be drawn out and detached from the printer body, and mounted on and attached to the printer body, as a integral photosensitive cartridge. Further, developing devices belonging to the photosensitive members are detachably mountable to the photosensitive cartridge.
Also known is a color image forming apparatus in which a plurality of developing machines are arranged correspondingly to respective colors and toner hoppers for supplementing toners to the developing machines are parallelly provided above side portions of the developing machines in a detachably mountable manner to the developing machines respectively.
However, each of the developing devices storing the toners corresponding to the respective colors must be increased in size in order to ensure sufficient volumes of the toners.
In the tandem-type color laser printer, on the other hand, the each developing device is attached/detached to/from the photosensitive cartridge. Therefore, if the developing devices are increased in size in order to ensure sufficient volumes of toners, the photosensitive cartridge must also be increased in size. Then, the printer body to/from which the photosensitive cartridge is attached/detached is inevitably increased in size.
In order to exchange each developing device in the tandem-type color laser printer, further, the photosensitive cartridge must be drawn out and detached from the printer body to exchange each developing device with a new one in the detached photosensitive cartridge, and must thereafter be mounted on and attached to the printer body again. In other words, the photosensitive cartridge including the photosensitive bodies, the coroner charger and the cleaner must be detached from and then attached to the printer body, in order to exchange each developing device. Thus, much labor is required for detaching the photosensitive cartridge from the printer body and thereafter attaching the photosensitive cartridge to the printer body.
Further, while each developing device storing the toner is provided with members such as a feed roller, a developing roller and a regulation blade necessary for developing a toner image, the developing device must be exchanged when the toner is used up, even if these members are still operable. Therefore, the running cost is increased.
The color laser printer can be undesirably increased in size due to the aforementioned structure including the plurality of photosensitive drums, the plurality of developing rollers and the plurality of toner boxes corresponding to the respective colors.
If the toner boxes are reduced in size in order to miniaturize the color laser printer, the volumes of the toners that can be accommodated in the toner boxes are reduced, so that the toner boxes must be frequently exchanged, leading to troublesome maintenance.
On the other hand, when sufficient volumes of toners are ensured in the toner boxes while miniaturizing the color laser printer, the toner boxes are limited in size reduction to inevitably occupy a large volume ratio in the color laser printer. Thus, arrangement of other parts in the color laser printer is restricted.
When the toner boxes are respectively provided with agitators for agitating the toners accommodated therein, for example, it is difficult to provide mechanisms for supplying driving force to the agitators on all the toner boxes in the limited space of the color laser printer.
When the developing machines and the toner hoppers are formed independently of each other as in the color image forming apparatus, for example, shutter members must be provided on all the toner hoppers respectively such that the toner hoppers communicate with the developing machines by opening the shutter members for feeding the toners to the developing machines and the toner hoppers are sealed by closing the shutter members for preventing the toners from leaking.
In this case, it is difficult to provide mechanisms for opening/closing the shutter members on all the toner hoppers in the limited space of the color laser printer.